1. Field of the Invention
This invention relates to the treatment of optical waveguide precursor materials to remove hydroxyl impurities therefrom.
2. Description of the Prior Art
The production of low-loss optical waveguides suitable for communications use is currently accomplished by a number of techniques, most of which comprise the oxidation of silicon tetrachloride to produce silica (SiO.sub.2). One method of manufacture is by the modified chemical vapor deposition technique (MDVD), such as is described in U. S. Pat. No. 4,217,027, assigned to the same assignee as the present invention.
It is known that --OH--containing impurities in optical fibers produce increased optical losses at various wavelengths of interest in optical communications systems. For example, --OH absorption peaks are located at 0.95, 1.25, and 1.39 microns, among others. While silicon tetrachloride, which is oxidized to produce silica, typically forms the bulk of the optical fiber precursor material, various amounts of dopants are also included. Typical dopants include germanium and phosphorus. These are typically present in a silica optical fiber in the form of their respective oxides, typically GEO.sub.2 and P.sub.2 O.sub.5. These are also typically incorporated by oxidizing a chloride of the above-noted materials, as by the MCVD process. For example, GeCl.sub.4 and POCl.sub.3 are typically oxidized along with the SiCl.sub.4 in varying amounts to produce the desired dopants in the optical fiber. Any --OH in the dopant chlorides also increases the amount of --OH in the optical fiber. It is known that the MCVD process partitions out some of the --OH-containing impurities in the oxidation process, reducing to some degree their incorporation in the optical fiber. Nevertheless, in the MCVD and certain other optical fiber production processes, it is desirable in some cases to reduce the amount of --OH impurities in the precursor materials.